Multilayer coatings, especially two-coat metallic finishes, are produced in particular by the basecoat/clearcoat method. This method is known and is described, for example, in U.S. Pat. No. 3,639,147 and EP-A-38 127.
The basecoat/clearcoat method can be used to produce finishes whose superiority over one-coat finishes is manifested in an enhanced effect and in the possibility of producing finishes with more brilliant and more clean colors.
The basecoat which is applied first of all determines, depending on the nature, quantity and spatial orientation of the pigments employed, the color and, if appropriate, the effect (e.g. metallic effect or pearl luster effect) of the finish.
Following application of the basecoat, at least some of the organic solvents and/or at least some of the water are or is removed from the applied basecoat film in an evaporation phase. A nonaqueous, transparent topcoat is then applied to this predried basecoat (wet-on-wet method). Basecoat and topcoat are then dried together.
The applied transparent topcoat gives the two-coat finish gloss and fullness and protects the pigmented coat applied from chemical and physical attack.
Using the method under discussion, it is only possible to obtain high-quality two-coat finishes if the transparent topcoat applied does not adversely affect the applied basecoat in such a way that the optical effect is impaired (e.g. clouding). On the other hand, the transparent topcoat must have a composition which ensures that it adheres well to the basecoat after the drying process. Further important properties which the transparent topcoat obtained after the drying process is required to have are a high degree of transparency, very good topcoat appearance, good gloss and good mechanical properties such as hardness, mar resistance and elasticity. Not least among the requirements, the transparent topcoat obtained after the drying process must have a high resistance to climatic effects (e.g. fluctuations in temperature, moisture in the form of water vapor, rain and dew, radiation-induced stress, etc.) and to attacks by acids or other chemicals such as, for example, organic solvents.
JP-A-1-158079 describes nonaqueous transparent topcoats for two-coat finishes of the basecoat/clearcoat type, which contain a hydroxyl group-containing polyacrylate resin which is obtainable by polymerizing from 10 to 50% by weight of an adduct of a cyclic ester, for example .epsilon.-caprolactone with hydroxyethyl acrylate or methacrylate, from 0 to 40% by weight of a hydroxyalkyl acrylate or methacrylate and from 30 to 80% by weight of a copolymerizable vinyl monomer to give a polyacrylate resin having a hydroxyl number of from 60 to 160, an acid number of from 0 to 40 and a glass transition temperature of from -50 to +40.degree. C. The transparent topcoats described in JP-A-1-158079 give finishes which are in need of improvement, especially with respect to their acid resistance and adhesion.
JP-A-4-1254 discloses coating compositions which contain, in addition to a crosslinking agent, a hydroxyl group-containing polyacrylate resin which has been prepared using 4-t-butylcyclohexyl acrylate and/or 4-t-butylcyclohexyl methacrylate as monomer component. The hydroxyl group-containing monomer employed for the preparation of the polyacrylate resin comprises, in particular, hydroxyethyl acrylate and hydroxyethyl methacrylate. These coating compositions known from JP-A-4-1254 have the particular disadvantage, when used as a transparent topcoat over a basecoat, that the resulting coatings are of inadequate adhesion to the basecoat. Furthermore, the resulting coatings have a poor solvent resistance, a high swellability and a poor overcoatability.
Finally, German Patent Application P 43 10 414.2, which is not a prior publication, describes coating compositions of the type described initially which contain as binder a hydroxyl group-containing polyacrylate resin which has been prepared using 4-hydroxy-n-butyl acrylate and/or 4-hyroxy-n-butyl methacrylate as monomer component. The coating compositions described therein are employed in particular in the sector of automotive production-line finishing. Coating compositions for the sector of automotive refinishing are not described in this application.
The object of the present invention was therefore to provide coating compositions which, when used as transparent topcoat over a basecoat, lead to coatings whose mar resistance is improved relative to known coating compositions. In addition, the resulting coatings should have in particular a good adhesion to the basecoat and, furthermore, a high degree of hardness coupled with good elasticity, a very good topcoat appearance, a high degree of transparency and good gloss. In addition, the resulting coatings should possess good polishability and good weathering resistance. Finally, the coating compositions should be readily processable and should be suitable for automotive refinishing; that is, they should be able to cure fully at low temperatures of in general below 120.degree. C., preferably below 80.degree. C. Even at these low temperatures the coating compositions should reach full cure rapidly (in particular, should display rapid through-drying) while nevertheless remaining processable (pot life) for as long as possible.
Surprisingly, this object is achieved by a coating composition of the type described initially, which is characterized in that
1.) component (A) is a hydroxyl group-containing polyacrylate resin which is obtainable by polymerizing
(a1) one or more monomers selected from the group consisting of 4-hydroxy-n-butyl acrylate and/or 4-hyroxy-n-butyl methacrylate and/or 3-hydroxy-n-butyl acrylate and/or 3-hydroxy-n-butyl methacrylate, and PA2 (a2) one or more monomers selected from the group consisting of 3-hydroxy-n-propyl acrylate and/or 3-hydroxy-n-propyl methacrylate and/or 2-hydroxy-n-propyl acrylate and/or 2-hydroxy-n-propyl methacrylate,
(b) from 0 to 20% by weight of a hydroxyl group-containing ester of acrylic acid or of methacrylic acid which is different from (a) and has at least 5 carbon atoms in the alcohol residue and/or of a hydroxyl group-containing ester of a polymerizable ethylenically unsaturated carboxylic acid, which is different from (a), or of a mixture of such monomers, PA0 (c) from 28 to 85% by weight of an aliphatic or cycloaliphatic ester of acrylic acid or of methacrylic acid which is different from (a) and (b) and has at least 4 carbon atoms in the alcohol residue, or of a mixture of such monomers, PA0 (d) from 0 to 25% by weight of an aromatic vinyl hydrocarbon which is different from (a), (b) and (c), or of a mixture of such monomers, PA0 (e) from 0 to 5% by weight of an ethylenically unsaturated carboxylic acid, or of a mixture of ethylenically unsaturated carboxylic acids, and PA0 (f) from 0 to 20% by weight of an ethylenically unsaturated monomer which is different from (a), (b), (c), (d) and (e), or of a mixture of such monomers PA0 (a) from 10 to 51% by weight, preferably from 10 to 35% by weight, of a mixture comprising PA0 (b) from 0 to 20% by weight, preferably from 0 to 10% by weight, of a hydroxyl group-containing ester of acrylic acid or of methacrylic acid which is different from (a) and has at least 5 carbon atoms in the alcohol residue and/or of a hydroxyl group-containing ester of a polymerizable ethylenically unsaturated carboxylic acid, which is different from (a), or of a mixture of such monomers, PA0 (c) from 28 to 85% by weight, preferably from 40 to 70% by weight, of an aliphatic or cycloaliphatic ester of acrylic acid or of methacrylic acid which is different from (a) and (b) and has at least 4 carbon atoms in the alcohol residue, or of a mixture of such monomers, PA0 (d) from 0 to 25% by weight, preferably from 5 to 20% by weight, of an aromatic vinyl hydrocarbon which is different from (a), (b) and (c), or of a mixture of such monomers, PA0 (e) from 0 to 5% by weight, preferably from 1 to 3% by weight, of an ethylenically unsaturated carboxylic acid, or of a mixture of ethylenically unsaturated carboxylic acids, and PA0 (f) from 0 to 20% by weight, preferably from 0 to 15% by weight, of an ethylenically unsaturated monomer which is different from (a), (b), (c), (d) and (e), or of a mixture of such monomers PA0 (a1) one or more monomers selected from the group consisting of 4-hydroxy-n-butyl acrylate and/or 4-hydroxy-n-butyl methacrylate and/or 3-hydroxy-n-butyl acrylate and/or 3-hydroxy-n-butyl methacrylate, and PA0 (a2) one or more monomers selected from the group consisting of 3-hydroxy-n-propyl acrylate and/or 3-hydroxy-n-propyl methacrylate and/or 2-hydroxy-n-propyl acrylate and/or 2-hydroxy-n-propyl methacrylate. PA0 (a1) from 10 to 85% by weight, preferably from 20 to 65% by weight, of component (a1), and PA0 (a2) from 15 to 90% by weight, preferably from 35 to 80% by weight, of component (a2), PA0 W.sub.n =proportion by weight of the nth monomer PA0 T.sub.Gn =glass transition temperature of the homopolymer of the nth monomer. PA0 The mixture which is employed in particular as component a) comprises PA0 (a1) 4-hydroxy-n-butyl acrylate and/or 3-hydroxy-n-butyl acrylate, and PA0 (a2) 3-hydroxy-n-propyl methacrylate and/or 2-hydroxy-n-propyl methacrylate. PA0 (B1) at least one polymer of an aliphatic and/or cycloaliphatic and/or of an araliphatic di- and/or polyisocyanate having an average functionality of from 3 to 4 and having a uretdione group content of not more than 5%, PA0 (B2) optionally at least one polymer of an aliphatic and/or cycloaliphatic and/or of an araliphatic di- and/or polyisocyanate having an average functionality of from 2 to 3 and having a uretdione group content of from 20 to 40%, and PA0 (B3) optionally at least one aliphatic and/or cycloaliphatic and/or araliphatic di- or poly isocyanate which is different from (B1) and (B2). PA0 (m.sub.1) from 5 to 80% by weight, preferably from 5 to 30% by weight, of a cycloaliphatic ester of methacrylic acid and/or acrylic acid, or of a mixture of such monomers, PA0 (m.sub.2) from 10 to 50% by weight, preferably from 15 to 40% by weight, of a hydroxyl group-containing alkyl ester of methacrylic acid and/or acrylic acid, or of a mixture of such monomers, PA0 (m.sub.3) from 0 to 25% by weight, preferably from 0to 15% by weight, of a hydroxyl group-containing ethylenically unsaturated monomer which is different from (m.sub.1) and (m.sub.2), or of a mixture of such monomers, PA0 (m.sub.4) from 5 to 80% by weight, preferably from 5 to 30% by weight, of an aliphatic ester of methacrylic and/or acrylic acid, which is different from (m.sub.1), (m.sub.2) and (m.sub.3), or of a mixture of such monomers, PA0 (m.sub.5) from 0 to 40% by weight, preferably from 10 to 30% by weight, of an aromatic vinyl hydrocarbon which is different from (m.sub.1), (m.sub.2), (m.sub.3) and (m.sub.4), or of a mixture of such monomers, and PA0 (m.sub.6) from 0 to 40% by weight, preferably from 0 to 30% by weight, of a further ethylenically unsaturated monomer which is different from (m.sub.1), (m.sub.2), (m.sub.3), (m.sub.4) and (m.sub.5), or of a mixture of such monomers, PA0 (1) a pigmented basecoat is applied to the substrate surface, PA0 (2) a polymer film is formed from the basecoat applied in step (1), PA0 (3) a transparent topcoat containing PA0 (4) basecoat and topcoat are cured together, characterized in that the topcoat employed is the coating composition according to the invention.
The present invention also relates to a process for producing a multilayer coating on a substrate surface, using these coating compositions, and to the use of the coating compositions in the sector of automotive refinishing.
It is surprising and was not foreseeable that the coating compositions according to the invention, when used as transparent topcoat over a basecoat, would be notable for very good mar resistance and very good adhesion to the basecoat. A further advantage is that the coating compositions lead to coatings having a high degree of hardness coupled with good elasticity, very good topcoat appearance, a high degree of transparency, good gloss, good polishability and high resistance to climatic effects (such as, for example, fluctuations in temperature, moisture in the form of water vapor, rain and dew, radiation-induced stress, etc.). Furthermore, the coating compositions have the advantage that they are readily processable and can be cured fully at low temperatures and can therefore be employed in the sector of automotive refinishing. Even when the coating compositions are cured at low temperatures, the coating compositions rapidly reach full cure while nevertheless remaining processable for a long time (pot life).